1. Field of the Invention
The present invention relates to a polypropylene resin composition having a high dielectric strength.
2. Discussion of the Prior Art
Polypropylene resins exhibit excellent mechanical properties, chemical properties, thermal properties, moldability and electrical properties and, therefore, have been used the manufacture of various industrial goods and products. Further, it is known that certain properties of polypropylene resins for example, rigidity, heat resistance, dimensional stability and resistance to marring can be improved by incorporating inorganic fillers into the polypropylene resin. The electrical properties of polypropylene resins, particularly, dielectric strength is improved by the incorparation of talc or clay in the polypropylene resin. Such resin compositions have been used in making parts, which have a high dielectric strength, such as a distributor's rotor of automobile engines.
Recently, there has been a demand in industry for polypropylene resin compositions having improved dielectric strength. For example, as a result of regulations in the automobile industry concerning exhaust emissions, a higher voltage must be applied to the rotor portion of a distributor. The resin compositions conventionally employed do not have a high enough dielectric strength to withstand such high voltages.
The present inventors previously proposed employing a polypropylene resin composition containing mica having a specific particle size distribution and an average particle size in a specified amount (J.P. KOKAI No. 62-273240) in applications in which a high dielectric strength is required. However, as shown in comparative Example I set forth herein, the proposed polypropylene resin composition exhibits excellent dielectric strength but has a low flowability and is very brittle. Thus, the resulting molded products are liable to crack and have defects, depending on their shape. As a result, the possible shapes of the molded products are limited, and the molded articles do not have a good appearance.